Coherent optical transmission technology is a technology in which a receiver includes a local optical oscillator, a beat signal that is generated through interference of received optical signal and local optical signal outputted from the local optical oscillator is converted into a signal of a baseband or an intermediate frequency band, and received equalized waveform is identified and regenerated, as with homodyne detection or heterodyne detection in wireless communication. The coherent optical transmission technology allows for improvement of reception sensitivity, compensation (delay equalization) of static dispersion in an optical fiber, and the like. On the other hand, issues such as synchronization in frequency/phase between the received optical signal and the local optical signal and polarization tracking remain.
A digital coherent optical transmission technology has been developed as a transmission technology that solves the above-described issues to realize transmission capacity higher than 100 Gbit/s per wavelength (for example, refer to NPL 1). In the digital coherent optical transmission technology, optical phase synchronization is performed by digital signal processing, and polarization mode dispersion of the optical fiber and delay characteristics deterioration by chromatic dispersion are adaptively compensated (adaptively equalized) to solve the issues of the conventional coherent optical transmission technology.
The digital coherent optical transmission technology makes it possible to perform flexible signal processing because digital signal processing is used for the above-described electric signal processing. In other words, it is possible to perform various processing such as error correction processing together in addition to the processing such as the optical phase synchronization and adaptive equalization described above. In addition, it becomes possible to use certain processing while determining propriety of the certain processing as necessary. For example, a technology in which one modulation system is optionally selected from different kinds of modulation systems according to instruction from outside has been developed.
A configuration and operation of a conventional optical transceiver apparatus that selects a multilevel modulation system from quadrature phase shift keying (QPSK) of 100 Gbit/s and 16 quadrature amplitude modulation (QAM) of 200 Gbit/s and performs transmission and reception, are described below.
FIG. 11 is a diagram illustrating the conventional optical transceiver apparatus that can optionally select the modulation system. The optical transceiver apparatus includes an optical transmitting section, an optical receiving section, a digital signal processing LSI, a framer LSI, and a frame transfer processing LSI. The optical transmitting section outputs an optical signal that has been modulated by the multilevel modulation system of QPSK or 16QAM. The optical receiving section receives the optical signal that has been modulated by the multilevel modulation of QPSK or 16QAM, and outputs an analog electric signal as a reception signal. The framer LSI performs reconstruction of a frame to be transmitted or received, and conversion of a frame format.
The optical transmitting section receives an electric modulation signal from the digital signal processing LSI, and outputs an optical signal that has been modulated by multilevel modulation system of QPSK or QAM. The optical transmitting section includes a laser diode (LD), a multilevel modulator, and a driver. The LD outputs laser light serving as a carrier wave. The multilevel modulator performs multilevel modulation on the laser light outputted from the LD. The driver drives the multilevel modulator. In a case of the QPSK modulation system of 100 Gbit/s, the optical transmitting section multiplexes input signals of 25 Gbit/s×total four lanes by four-level phase multiplexing (×2) and polarization multiplexing (×2), thereby achieving a transmission rate of 100 Gbit/s per wavelength. In addition, in a case of the 16QAM modulation system of 200 Gbit/s, the optical transmitting section multiplexes input signals of 25 Gbit/s×total eight lanes by 16-level phase multiplexing (×4) and polarization multiplexing (×2), thereby achieving a transmission rate of 200 Gbit/s per wavelength.
The optical receiving section receives the multilevel-modulated optical signal, and outputs an analog electric signal serving as a reception signal. The optical receiving section includes a local optical oscillator (LO), a 90-degree optical hybrid circuit, and a balanced photodetector (PD) array. In a case of the optical receiving section of the QPSK modulation system, the balanced PD array outputs signals of total four lanes (two pairs of IQ signals). In a case of the optical receiving section of the 16QAM modulation system, the balanced PD array outputs signals of total eight lanes (four pairs of IQ signals).
Note that the optical transmitting section and the optical receiving section may be integrally mounted as an optical transceiver in some cases. As the optical transceiver, an analog pluggable form such as CFP2-ACO is also used.
The digital signal processing LSI converts the analog reception signal into a digital signal, and demodulates the reception signal through digital signal processing. Further, the digital signal processing LSI encodes the signal to be transmitted, into a modulated signal corresponding to various kinds of modulation system (QPSK or 16QAM).
The configuration and operation of the digital signal processing LSI are described in detail. The analog reception signal provided from the optical receiving section is converted into a digital signal by an analog-digital (AD) convertor. The digital signal provided from the AD convertor is subjected to chromatic dispersion compensation by a chromatic dispersion compensation section, and is subjected to waveform equalization by an adaptive equalization section, and is then waveform equalized and regenerated by a demodulation section. The chromatic dispersion compensation section compensates chromatic dispersion that is static dispersion of the optical fiber serving as a transmission path. The adaptive equalization section adaptively compensates high-speed waveform deterioration that is mainly caused by polarization fluctuation of the optical signal transmitted through the optical fiber.
These processes are performed by the digital signal processing but it is difficult to serially process the high-speed signal of 25 Gbit/s/lane outputted from the optical receiving section. Therefore, the high-speed signal is typically converted into a parallel signal of about several hundred Mbit/s/lane by the AD convertor, and the digital signal processing is then performed on the parallel signal. The regenerated reception signal is converted into a serial signal of 25 Gbit/s/lane by a parallel-serial convertor of an input/output interface section. The serial signal is then provided as an electric signal of 100 Gbit/s (25 Gbit/s×four lanes), to the framer LSI.
In contrast, a signal to be transmitted (hereinafter, referred to as a transmission signal) that is outputted from the framer LSI is converted, by a serial-parallel convertor of the input/output interface section, into a parallel signal that is suitable for the digital signal processing. The parallel transmission signal is then encoded by a modulation section. The encoded transmission signal is converted, by a digital-analog (DA) convertor, into an analog modulation signal of 25 Gbit/s/lane for driving of the multilevel modulator, and the analog modulation signal is provided to the optical transmitting section.
The chromatic dispersion compensation section, the adaptive equalization section, the demodulation section, and the modulation section described above are collectively referred to as a signal processing section. Note that the signal processing section may further include a function section that performs processing such as error correction processing other than those described above in some cases.
The digital signal processing LSI includes two pairs of input/output interface sections A and B that correspond to two pairs of transmission signals of 100 Gbit/s. When the optical transceiver apparatus transmits and receives an optical signal of 200 Gbit/s modulated by 16QAM, the two pairs of transmission signals of 100 Gbit/s (for example, OTU4) are provided to the signal processing section and are multiplexed.
Note that the function executed by the framer LSI may be mounted on the digital signal processing LSI, as one function of the signal processing section. In this case, the input/output interface sections of FIG. 11 are each directly connected to the frame transfer processing LSI.
Next, a configuration and operation of an optical transceiver apparatus that uses the digital signal processing LSI mounted with both of the QPSK modulation system and the 16QAM modulation system illustrated in FIG. 11 so as to be applicable to both transmission applications of 100 Gbit/s×two wavelengths and 200 Gbit/s×one wavelength, are described with reference to FIGS. 12 and 13.
FIG. 12 is a diagram illustrating a conventional optical transceiver apparatus in a case where each of two optical transceivers transmits and receives an optical signal of a bit rate of 100 Gbit/s by the QPSK modulation system. Optical transceivers 1a and 1b use the optical signals with different wavelengths, which makes it possible to transmit the optical signals of 100 Gbit/s×2 wavelengths=200 Gbit/s by wavelength multiplexing.
The optical transceivers 1a and 1b and the digital signal processing LSIs 2a and 2b in FIG. 12 have configurations same as the respective configurations of the optical transceiver and the digital signal processing LSI illustrated in FIG. 11. A frame processing section 3 includes the framer LSI and the frame transfer processing LSI, and includes at least three ports P1 to P3 that can exchange signals with the input/output interface section provided in each of the digital signal processing LSIs 2a and 2b. Input/output interface sections A and B of the digital signal processing LSI 2a and an input/output interface section C of the digital signal processing LSI 2b are respectively electrically connected to the ports P1 to P3 of the frame processing section 3. Note that, in this example, the input/output interface sections and the ports P1 to P3 of the frame processing section 3 are connected to one another through four lanes in both of transmission and reception.
The optical transceiver 1a transmits and receives the optical signal of 100 Gbit/s that has been modulated by QPSK modulation system. It is sufficient for the digital signal processing LSI 2a to operate only the input/output interface section A as the interface with the frame processing section 3 because the digital signal processing LSI 2a only processes the signal of 100 Gbit/s. Accordingly, the input/output interface section B is electrically connected to the port P2 of the frame processing section 3 for transmission application of 200 Gbit/s×one wavelength described later but does not exchange signal in the transmission of 100 Gbit/s×two wavelengths. Likewise, the optical transceiver 1b transmits and receives the optical signal of 100 Gbit/s that has been modulated by QPSK modulation system. The digital signal processing LSI 2b processes the signal of 100 Gbit/s that is transmitted and received by the optical transceiver 1b, and exchanges the signal with the port P3 of the frame processing section 3 through the input/output interface section C. Operation of the input/output interface section D is unnecessary.
FIG. 13 is a diagram illustrating a conventional optical transceiver apparatus in a case where a single optical transceiver transmits and receives an optical signal at a bit rate of 200 Gbit/s by 16QAM modulation system. The configuration and the connection form of the optical transceiver apparatus are similar to those of the optical transceiver apparatus of FIG. 12 except that not two optical transceivers but one optical transceiver is provided. Since the optical transceiver 1a transmits and receives the optical signal of 200 Gbit/s that has been modulated by 16QAM modulation system, however, it is necessary to operate the two input/output interface sections A and B of the digital signal processing LSI 2a. In addition, the input/output interface sections A and B of the digital signal processing LSI 2a exchange the signal with the ports P1 and P2 of the frame processing section 3, respectively. On the other hand, operation of the digital signal processing LSI 2b is unnecessary. The input/output interface section C of the digital signal processing LSI 2b is electrically connected to the port P3 of the frame processing section 3 but does not exchange a signal.
A tradeoff is present in both of the above-described applications of “transmission of 100 Gbit/s×two wavelengths” by the QPSK modulation system and “transmission of 200 Gbit/s×one wavelength” by the 16QAM modulation system. The 16QAM modulation system is large in data amount (bit number) transmitted and received by one symbol as compared with the QPSK modulation system but is short in transmittable distance as compared with the QPSK modulation system. In other words, in the “transmission of 200 Gbit/s×one wavelength”, it is possible to perform optical signal transmission of 200 Gbit/s by one optical transceiver but the transmittable distance is small. On the other hand, in the “transmission of 100 Gbit/s×two wavelengths”, it is possible to obtain large transmittable distance but two optical transceivers and two optical wavelength resources are necessary. Each of the optical transceiver apparatuses illustrated in FIGS. 12 and 13 is selectively applicable to one of applications of the “transmission of 100 Gbit/s×two wavelengths” by the QPSK modulation system and the “transmission of 200 Gbit/s×one wavelength” by the 16QAM modulation system, by taking into consideration the characteristics (such as loss and chromatic dispersion) of the optical fiber serving as the transmission medium, a usable wavelength resource, an apparatus cost, and the like.